The present invention relates to the field of light emitting devices, in particular, to organic semiconductor lasers.
Several recent publications have reported either superluminescence or amplified spontaneous emission in is polymeric organic light emitters such as conjugated polymers. (N. Tessier et al., Nature 382, 695 (1996); F. Hide et al., Science 273, 1833 (1996)). The materials used in those emitters were spin-coated from a solution of the polymer or its chemical precursors. Optically pumped, stimulated emission from organic laser dyes, introduced into inert, spin-coated polymers or gels has been described in the literature. (R. E. Hermes, et al., Appl. Phys. Lett. 63, 877 (1993); M. N. Weiss et al., Appl. Phys. Lett. 69, 3653 (1996); H. Kogelnik et al., Appl. Phys. Lett. 18, 152 (1971); M. Canva et al., Appl. Opt., 34, 428 (1995)).
Recent work has demonstrated gain-narrowed photoluminescence spectra with full widths at half maxima (FWHM) of 40-60 xc3x85 in response to a short pulse laser excitation, typically 1 xcexcJ in a 10 ns pulse. (Materials Research Society 1997 Spring Meeting, Abstracts H1.1, H1.6, H2.1, H2.2, H2.3.) Such work is potentially applicable to electrically pumped organic solid state lasers xe2x80x9cplastic lasersxe2x80x9d). If realized, such devices could offer low cost and ease of integration of laser sources onto either conventional semiconductor circuitry or lightweight plastic substrates.
Spun-on polymeric materials, however, do not exhibit particularly good thickness uniformity, ability to achieve extremely high materials purity, and ease of integration with other conventional semiconductor fabrication processes.
In the field of organic light emitting devices (OLEDs) for flat panel display applications, small molecule OLEDs currently offer better operating lifetimes by an order of magnitude over their spin-coated, polymeric analogs. (L. J.Rothberg et al., xe2x80x9cStatus of and Prospects for Organic Electroluminescencexe2x80x9d, J. Mater. Res. 1996, 11:3174; N. C. Greenham et al., xe2x80x9cSemiconductor Physics of Conjugated Polymersxe2x80x9d, Solid State Physics 1995, 49:1.)
However, there has been no known demonstration of laser action in a vacuum-deposited organic thin film structure. Furthermore, there is considerable skepticism about the realization of small-molecule organic lasers because of quenching processes which can occur in such materials. Such quenching processes are observed at high carrier densities and lead to decreased photoluminescence quantum efficiency. For example, bimolecular reactions in Alq3 films have been found to cause the quantum efficiency of photoluminescence to begin to decrease at incident intensities above 1014 photons/cm2. (D. Y. Zang et al., Appl. Phys. Lett. 60 (2), 189, 1992.)
The present invention is directed to a small-molecule, organic thin film laser with very low threshold lasing. Both optically and electrically pumped embodiments are disclosed.
In contrast to spun-on polymeric materials, vacuum-deposition of small molecular weight organic materials offers the advantages of excellent thickness uniformity, extremely high materials purity, and ease of integration with other conventional semiconductor fabrication processes.
In an exemplary embodiment of the present invention, very low threshold, optically-pumped lasing is achieved in a vacuum-deposited, organic thin film comprising a layer of tris(8-hydroxyquinoline) aluminum (Alq3) doped with DCM laser dye. A very low lasing threshold is achieved at a pump energy density of 1.5 xcexcJ/cm2 with a 500 psec excitation pulse. Above the threshold, several extremely narrow (i.e., less than 1 xc3x85 FWHM), linearly polarized Fabry-Perot modes appear in the output spectrum. The peak output power above the threshold exceeds 30 W from a 3xc3x9710xe2x88x927 cm2 output facet, corresponding to a peak power of approximately 108 W/cm2.
Bright red laser emission is clearly visible from the edge of the device. The output laser beam includes several transverse modes which diverge in a direction orthogonal to the surface of the device of the present invention. The emission is strongly linearly polarized, as one would expect for laser emission. No appreciable degradation of laser material occurs after several hours of pulsed operation in a dry nitrogen atmosphere.
The present invention provides a laser device with a small-molecule, vacuum-depositable organic thin film which exhibits a low lasing threshold (1.5 xcexcJ/cm2), high efficiency, narrow line width (less than 1 xc3x85) and high peak power (30 W). The pump threshold corresponds to a current density of 10-50 A/cm2 for an electrically pumped laser using such materials.
The ease of processing, low threshold and other characteristics of vacuum-deposited materials opens the door to an entirely new generation of optically and electrically-pumped solid-state lasers using vacuum-deposited organic semiconductors.
The laser of the present invention can be used in a wide variety of applications, including telecommunications, printing, optical downconversion, semiconductor circuit etching, thermal processing (e.g., marking, soldering and welding), spectroscopy, vehicular control and navigation, measurement devices, optical memory devices, displays, scanners, pointers, games and entertainment systems and sensors.